1. Field of the Invention
The present invention relates to a method for producing a semiconductor film and a semiconductor device having the semiconductor film. In particular, the present invention relates to a method for producing a semiconductor film and a thin film transistor used for an active matrix type liquid crystal display device, an image sensor, and the like.
2. Description of the Related Art
Active matrix type liquid crystal display devices and the like use thin film transistors (TFTs) for driving pixels provided therein. Each TFT generally includes a silicon semiconductor film formed on an insulating substrate made of glass or the like; therefore, the characteristics of the TFT greatly depend upon those of the silicon semiconductor film.
A silicon semiconductor film falls roughly into two categories: an amorphous silicon (a-Si) semiconductor film and a crystalline silicon semiconductor film.
The amorphous silicon semiconductor film has generally been used, since it can be relatively easily produced by a vapor phase method at lower temperatures. In spite of this mass-productivity of the amorphous silicon semiconductor film, the physical properties thereof, such as conductivity, are inferior to those of the crystalline silicon semiconductor film. Thus, in order to obtain a silicon semiconductor film provided with higher electron-mobility, there has been strong demand for the establishment of a method for producing a TFT made of a crystalline silicon semiconductor film.
Examples of the material used for the crystalline silicon semiconductor film include polycrystalline silicon, micro-crystalline silicon, amorphous silicon containing a crystalline component, and semi-amorphous silicon having an intermediate state between crystallinity and non-crystallinity. As a method for obtaining a thin film shaped silicon semiconductor having these crystallinities, the following three methods are known.
(1) A crystalline silicon semiconductor film is directly formed.
(2) An amorphous silicon film is formed, and then is crystallized by the irradiation of a laser beam or the like.
(3) An amorphous silicon film is formed, and then is crystallized with heat energy.
These methods have the following problems.
According to method (1), the film is crystallized while being formed. For this reason, a silicon film with large thickness is required to be formed so as to obtain crystalline silicon with a large particle size. However, it is technologically difficult to uniformly form a film having satisfactory semiconductor physical properties over an entire surface of a substrate. Furthermore, in this case, a temperature for forming the film is 600.degree. C. or higher, causing a problem in terms of cost, that is, the impossibility of using an inexpensive glass substrate having a low softening point.
According to method (2), a crystallization phenomenon is utilized in a process during which a film is changed from a liquid state to a solid state; therefore, the grain boundaries are satisfactorily treated even though the particle size of the crystal is small. Thus, a crystalline silicon film of high quality can be obtained. In spite of this advantage, in the case of an excimer laser which has generally been used, an area to be irradiated with a laser beam is small, so that throughput is low. In addition, the stability of the excimer laser is not sufficient for uniformly treating the entire surface of a large substrate.
Method (3) has an advantage of being applied to a large area, unlike methods (1) and (2). However, method (3) requires heat treatment at a high temperature of 600.degree. C. or more over tens of hours during crystallization. Thus, in order to use an inexpensive glass substrate and improve throughput, it is required to lower a heating temperature and crystallize the film for a short period of time. In addition, since method (3) utilizes a solid phase crystallization phenomenon, crystal particles are grown by spreading in parallel with the substrate surface to form particles having a particle size of several .mu.m, and crystal particles thus grown come into contact with each other to form grain boundaries. These grain boundaries become lattice defects and work as a trap level trapping carriers, causing a great decrease in electron-mobility of TFTs.
In order to solve the above-mentioned problems of the crystal particle boundaries in method (3), the following two methods (4) and (5) have been proposed.
According to method (4), impurities such as silicon (Si.sup.+) are introduced by an ion implantation method or the like, and then a polycrystalline silicon film having a crystal particle size of several .mu.m is obtained by heat treatment (Japanese Laid-Open Patent Publication No. 5-55142).
According to method (5), silicon particles having a particle size of 10 to 100 nm are sprayed onto an amorphous silicon film together with high-pressure nitrogen gas to form growth cores (Japanese Laid-Open Patent Publication No. 5-136048).
In both of methods (4) and (5), foreign particles which are to become growth cores of crystal are selectively introduced into a predetermined region of the amorphous silicon film, and then crystal growth is allowed to be effected using the foreign particles as the cores by heat treatment to obtain a crystalline silicon film of high quality having a large particle size. Accordingly, a semiconductor device such as a TFT is produced by utilizing the crystalline silicon film thus obtained.
However, according to methods (4) and (5), the introduced foreign particles work only as crystal cores. Therefore, although the foreign particles are effective for the generation of crystal cores and the control of a crystal growth direction, the above-mentioned problems in the step of heat treatment for crystallization remains unresolved. For example, in method (4), the crystallization of the film is effected by heat treatment at 600.degree. C. for 40 hours, and in method (5), the crystallization of the film is effected by heat treatment at 650.degree. C. or higher. Both of these methods (4) and (5) use such a high temperature for heat treatment, so that these methods are effective for an SOI substrate and an SOS substrate but are not appropriate for the application to an inexpensive glass substrate. For example, glass of Corning 7059 used for an active matrix type liquid crystal display device has a glass strain point of 593.degree. C. Therefore, when the liquid crystal display device having a large area is produced by using this glass as a substrate, heating at 600.degree. C. or higher causes strain of the substrate.
The present invention overcomes the above-mentioned conventional problems, and its objective is to provide a method for producing a crystalline silicon film, in which the crystallization is accelerated by heat treatment at low temperature for a short period of time and the effect of the crystal boundaries is reduced. More specifically, the objective of the present invention is to provide a method for uniformly producing a semiconductor thin film having crystallinity on the surface of the substrate by effecting the crystallization at a temperature of 600.degree. C. or lower for a short period of time. Furthermore, according to the present invention, crystallinity higher than that obtained by a conventional heat treatment is obtained, and the concentration of catalytic elements contained in a semiconductor thin film having crystallinity is reduced, thereby providing a method for producing a semiconductor film of high performance which is excellent in reliability and electrical stability and a method for producing a semiconductor device using such a semiconductor film.